1. Field of the Invention
The invention relates to implantable devices or endoluminal prostheses, such as stents, and methods of coating such devices.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially press against the atherosclerotic plaque of the lesion for remodeling of the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient""s vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings, which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may necessitate another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable, intraluminal prosthesis, one example of which is a stent, is implanted in the lumen to maintain the vascular patency.
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents that have been applied in PTCA procedures include U.S. Pat. No. 4,733,665 issued to Pahnaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis is still a significant clinical problem with rates ranging from 20-40%. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or even toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
Depending on the physiological mechanism targeted, the therapeutic substance may be required to be released at an efficacious concentration for an extended duration of time. Increasing the quantity of the therapeutic substance in the polymeric coating can lead to poor coating mechanical properties, inadequate coating adhesion, and overly rapid rate of release. Increasing the quantity of the polymeric compound by producing a thicker coating can perturb the geometrical and mechanical functionality of the stent, as well as limit the procedure for which the stent can be used.
It is desirable to increase the residence time of a substance at the site of implantation, at a therapeutically useful concentration, without the application of a thicker coating. It is also desirable to be able to increase the quantity of the therapeutic substance carried by the polymeric layer without perturbing the mechanical properties of the coating, such as adhesion of the polymer to the stent substrate.
The present invention provides a method of forming a coating for a prosthesis, e.g., a stent. The method includes depositing a polymeric sheath over at least a portion of a prosthesis. The prosthesis has a plurality of interconnected struts separated by gaps and a longitudinally extending central bore for allowing a fluid to travel through the prosthesis. The method further includes exposing the polymeric sheath to a temperature not greater than about the melting temperature of the polymer to form a coating for the prosthesis. The method can further include removing a portion of the coating positioned over some of the gaps to form a pattern of interstices dispersed between the struts for allowing a fluid that flows through the central bore to seep through the coating.
In one embodiment, the coating contains an active ingredient. In other embodiments, the coating contains radiopaque elements or radioactive isotopes.
Also provided is a method for increasing an amount of a polymeric coating on a stent having struts separated by gaps, without increasing the thickness of the coating. The method includes inserting a stent having a plurality of interconnected struts separated by gaps into a polymeric sheath. The method further includes exposing the polymeric sheath to a temperature not greater than about the melting temperature of the polymer to form a coating for the stent. The coating covers the struts and the gaps between the struts so as to increase the quantity of the coating supported by the stent without increasing the thickness of the coating on the stent. The method can also include removing a portion of the coating deposited over at least one of the gaps to create an opening in the coating. The size of the opening is smaller than the size of the gap. The opening allows a fluid, such as blood, to travel through the coating from within the stent.